Sample Size and Power
Laura Lee Johnson, Ph.D.
Statistician
Chapter 22, 3rd Edition
Chapter 15, 2nd Edition
Objectives
• Calculate changes in sample size based on
changes in the difference of interest, variance,
or number of study arms
• Understand intuition behind power
calculations
• Recognize sample size formulas for the tests
• Learn tips for getting through an IRB
Take Away Message
• Get some input from a statistician
– This part of the design is vital and mistakes can be
costly!
• Take all calculations with a few grains of salt
– “Fudge factor” is important!
• Round UP, never down (ceiling)
– Up means 10.01 becomes 11
• Analysis Follows Design
Take Home: What you need for N
• What difference is scientifically important in units –
thought, discussion
– 0.01 inches?
– 10 mm Hg in systolic blood pressure?
• How variable are the measurements (accuracy)? –
Pilot!
– Plastic ruler, Micrometer, Caliper
Sample Size
• Difference (effect) to be detected (δ)
• Variation in the outcome (σ2)
• Significance level (α)
– One-tailed vs. two-tailed tests
• Power
• Equal/unequal arms
• Superiority or equivalence or non-inferiority
Vocabulary
• Follow-up period
– How long a participant is followed
• Censored
– Participant is no longer followed
• Incomplete follow-up (common)
• Administratively censored (end of study)
• More in our next lecture
Outline
Power
• Basic Sample Size Information
• Examples (see text for more)
• Changes to the basic formula
• Multiple comparisons
• Poor proposal sample size statements
• Conclusion and Resources
Power Depends on Sample Size
• Power = 1-β = P( reject H0 | H1 true )
– “Probability of rejecting the null hypothesis if the
alternative hypothesis is true.”
• More subjects  higher power
Power is Affected by…..
• Variation in the outcome (σ2)
– ↓ σ2
→ power ↑
• Significance level (α)
–↑α
→ power ↑
• Difference (effect) to be detected (δ)
–↑δ
→ power ↑
• One-tailed vs. two-tailed tests
– Power is greater in one-tailed tests than in
comparable two-tailed tests
Power Changes
• 2n = 32, 2 sample test, 81% power, δ=2, σ = 2,
α = 0.05, 2-sided test
• Variance/Standard deviation
– σ: 2 → 1 Power: 81% → 99.99%
– σ: 2 → 3 Power: 81% → 47%
• Significance level (α)
– α : 0.05 → 0.01 Power: 81% → 69%
– α : 0.05 → 0.10 Power: 81% → 94%
Power Changes
• 2n = 32, 2 sample test, 81% power, δ=2, σ = 2,
α = 0.05, 2-sided test
• Difference to be detected (δ)
– δ : 2 → 1 Power: 81% → 29%
– δ : 2 → 3 Power: 81% → 99%
• Sample size (n)
– n: 32 → 64 Power: 81% → 98%
– n: 32 → 28 Power: 81% → 75%
• Two-tailed vs. One-tailed tests
– Power: 81% → 88%
Power should be….?
• Phase III: industry minimum = 80%
• Some say Type I error = Type II error
• Many large “definitive” studies have power
around 99.9%
• Proteomics/genomics studies: aim for high
power because Type II error a bear!
Power Formula
• Depends on study design
• Not hard, but can be VERY algebra intensive
• May want to use a computer program or
statistician
Outline
Power
Basic Sample Size Information
• Examples (see text for more)
• Changes to the basic formula
• Multiple comparisons
• Rejected sample size statements
• Conclusion and Resources
Basic Sample Size
• Changes in the difference of interest have HUGE
impacts on sample size
– 20 point difference → 25 patients/group
– 10 point difference → 100 patients/group
– 5 point difference → 400 patients/group
• Changes in difference to be detected, α, β, σ, number
of samples, if it is a 1- or 2-sided test can all have a
large impact on your sample size calculation
4( Z 1   / 2  Z 1   ) 
Basic 2-Arm Study’s
TOTAL Sample Size = 2 N 
2
2

2
Basic Sample Size Information
• What to think about before talking to a
statistician
• What information to take to a statistician
– In addition to the background to the project
Nonrandomized?
• Non-randomized studies looking for
differences or associations
– Require larger sample to allow adjustment for
confounding factors
• Absolute sample size is of interest
– Surveys sometimes take % of population approach
Comments
• Study’s primary outcome
– Basis for sample size calculation
– Secondary outcome variables considered
important? Make sure sample size is sufficient
• Increase the ‘real’ sample size to reflect loss to follow
up, expected response rate, lack of compliance, etc.
– Make the link between the calculation and
increase
• Always round up
– Sample size = 10.01; need 11 people
Sample Size in Clinical Trials
•
•
•
•
Two groups
Continuous outcome
Mean difference
Similar ideas hold for other outcomes
Sample Size Formula Information
• Variables of interest
– type of data e.g. continuous, categorical
•
•
•
•
Desired power
Desired significance level
Effect/difference of clinical importance
Standard deviations of continuous outcome
variables
• One or two-sided tests
Sample Size & Data Structure
•
•
•
•
•
•
Paired data
Repeated measures
Groups of equal sizes
Hierarchical or nested data
Biomarkers
Validity (of what) studies
Sample Size & Study Design
•
•
•
•
•
•
•
Randomized controlled trial (RCT)
Block/stratified-block randomized trial
Equivalence, non-inferiority, superiority trial
Non-randomized intervention study
Observational study
Prevalence study
Measuring sensitivity and specificity
Outline
Power
Basic sample size information
Examples (see text for more)
• Changes to the basic formula
• Multiple comparisons
• Rejected sample size statements
• Conclusion and Resources
How many humans do I need? Short
Helpful Hints
• Not about power, about stability of estimates
• 15/arm minimum: good rule of thumb for early
studies
– 12-15 gives somewhat stable variance, sometimes
– If using Bayesian analysis techniques at least
70/arm
• If n < 20-30, check t-distribution
• Minimum 10 participants/variable
– Maybe 100 per variable
Live Statistical Consult!
• Sample size/Power calculation: cholesterol in
hypertensive men example (Hypothesis
Testing lecture)
• Choose your study design
– Data on 25 hypertensive men (mean 220, s=38.6)
– 20-74 year old male population: mean serum
cholesterol is 211 mg/ml with a standard
deviation of 46 mg/ml
Example
• Calculate power with the numbers given
• What is the power to see a 19 point difference
in mean cholesterol with 25 people in
– Was it a single sample or 2 sample example?
Sample Size Rulers
JAVA Sample Size
Put in 1-Sample Example #s
•
•
•
•
•
1 arm, t-test
Sigma (sd) = 38.6
True difference of means = 220-211=9
n=25
2 sided (tailed) alpha = 0.05
– Power=XXXX
• 90% power
– Solve for sample size n=XXXX
Move the Values Around
• Sigma (standard deviation, sd)
• Difference between the means
Put in 2-Sample Example #s
•
•
•
•
•
•
2 arms, t-test
Equal sigma (sd) in each arm = 2
2 sided (tailed) alpha = 0.05
True difference of means = 1
90% power
Solve for sample size
Keep Clicking “OK” Buttons
Phase I: Dose Escalation
• Dose limiting toxicity (DLT) must be defined
• Decide a few dose levels (e.g. 4)
• At least three patients will be treated on each
dose level (cohort)
• Not a power or sample size calculation issue
Phase I (Old Way)
• Enroll 3 patients
• If 0 out of 3 patients develop DLT
– Escalate to new dose
• If DLT is observed in 1 of 3 patients
– Expand cohort to 6
– Escalate if 0 out of the 3 new patients do not
develop DLT (i.e. 1/6 at that dose develop DLT)
Phase I (cont.)
• Maximum Tolerated Dose (MTD)
– Dose level immediately below the level at which
≥2 patients in a cohort of 3 to 6 patients
experienced a DLT
• Usually go for “safe dose”
– MTD or a maximum dosage that is pre-specified in
the protocol
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Phase I
Enroll 3 people
0/3 DLT
1/3 DLT
Escalate to
new dose
Enroll 3 more
at same dose
0/new 3 DLT
Escalate to
new dose
2 or 3 / 3 DLT
Stop Drop
down
dose;
start
1 or more /
over
new 3 DLT
Stop
Number of pts with DLT
Decision
0/3
Escalate one level
1/3
Enroll 3 more at current level
0/3 + 0/3
STOP and choose current level as MTD
(To get here a de-escalation rule
must have been applied at the
next higher dose level)
1/3 + 0/3
Escalate one level
(unless a de-escalation rule was applied at
next higher level, in which case choose
current level as MTD)
1/3 + {1/3* or 2/3 or 3/3}
STOP* and choose previous level as
MTD
(unless previous level has only 3 patients, in
which case treat 3 more at previous level)
2/3 or 3/3
STOP and choose previous level as MTD
(unless previous level has only 3 patients, in
which case treat 3 more at previous level)
Phase I Note
• *Implicitly targets a dose with Pr (Toxicity) ≤
0.17; if at 1/3+1/3 decide current level is MTD
then the Pr (Toxicity) ≤ 0.33
• Entry of patients to a new dose level does not
occur until all patients in the previous level are
beyond a certain time frame where you look
for toxicity
• Not a power or sample size calculation issue
Phase I
•
•
•
•
MANY new methods
Several randomize to multiple arms
Several have control arms
Several have 6-15 people per arm
Phase II Designs
• Screening of new therapies
• Not to prove ‘final’ efficacy, usually
– Efficacy based on surrogate outcome
• Sufficient activity to be tested in a randomized
study
• Issues of safety still important
• Small number of patients (still may be in the
hundreds total, but maybe less than 100/arm)
Phase II Design Problems
• Might be unblinded or single blinded
treatment
• Placebo effect
• Investigator bias
• Regression to the mean
Phase II:
Two-Stage Optimal Design
• Seek to rule out undesirably low response
probability
– E.g. only 20% respond (p0=0.20)
• Seek to rule out p0 in favor of p1; shows
“useful” activity
– E.g. 40% are stable (p1=0.40)
Phase II Example:
Two-Stage Optimal Design
• Single arm, two stage, using an optimal design
& predefined response
• Rule out response probability of 20% (H0:
p=0.20)
• Level that demonstrates useful activity is 40%
(H1:p=0.40)
• α = 0.10, β = 0.10
Two-Stage Optimal Design
• Let α = 0.1 (10% probability of accepting a
poor agent)
• Let β = 0.1 (10% probability of rejecting a good
agent)
• Charts in Simon (1989) paper with different p1
– p0 amounts and varying α and β values
Table from Simon (1989)
Blow up: Simon (1989) Table
Phase II Example
• Initially enroll 17 patients.
– 0-3 of the 17 have a clinical response then stop
accrual and assume not an active agent
• If ≥ 4/17 respond, then accrual will continue
to 37 patients
Phase II Example
• If 4-10 of the 37 respond this is insufficient
activity to continue
• If ≥ 11/37 respond then the agent will be
considered active
• Under this design if the null hypothesis were
true (20% response probability) there is a 55%
probability of early termination
Sample Size Differences
• If the null hypothesis (H0) is true
• Using two-stage optimal design
– On average 26 subjects enrolled
• Using a 1-sample test of proportions
– 34 patients
– If feasible
• Using a 2-sample randomized test of
proportions
– 86 patients per group
Phase II
• Newer methods are available
• Many cite Simon (thus, why we went through
it)
Phase II: Historical Controls
• Want to double disease X survival from 15.7
months to 31 months.
• α = 0.05, one tailed, β = 0.20
• Need 60 patients, about 30 in each of 2 arms;
can accrue 1/month
• Need 36 months of follow-up
• Use historical controls
Phase II: Historical Controls
• Old data set from 35 patients treated at NCI
with disease X, initially treated from 1980 to
1999
• Currently 3 of 35 patients alive
• Median survival time for historical patients is
15.7 months
• Almost like an observational study
• Use Dixon and Simon (1988) method for
analysis
Phase II Summary
Study
Design
1 arm
Advantages
Disadvantages
Small n
No control
1 arm
2-stage
Small n, stop
early
Historical
controls
2(+) arm
Small n, some
control
Control
No control,
correct
responder/non
responder rules
Accurate control
?
Larger n
8 arm
?
?
Phase III Survival Example
• Primary objective: determine if patients with
metastatic melanoma who undergo Procedure
A have a different overall survival compared
with patients receiving standard of care (SOC)
• Trial is a two arm randomized phase III single
institution trial
Number of Patients to Enroll?
• 1:1 ratio between the two arms
• 80% power to detect a difference between 8
month median survival and 16 month median
survival
• Two-tailed α = 0.05
• 24 months of follow-up after the last patient
has been enrolled
• 36 months of accrual
3
4
1
3
1
2
Phase III Survival
• Look at nomograms (Schoenfeld and Richter).
Can use formulas
• Need 38/arm, so let’s try to recruit 42/arm –
total of 84 patients
• Anticipate approximately 30 patients/year
entering the trial
Non-Survival Simple Sample Size
• Start with 1-arm or 1-sample study
• Move to 2-arm study
• Study with 3+ arms cheat trick
– Calculate PER ARM sample size for 2-arm study
– Use that PER ARM
– Does not always work; typically ok
1-Sample N Example
• Study effect of new sleep aid
• 1 sample test
• Baseline to sleep time after taking the
medication for one week
• Two-sided test, α = 0.05, power = 90%
• Difference = 1 (4 hours of sleep to 5)
• Standard deviation = 2 hr
Sleep Aid Example
•
•
•
•
1 sample test
2-sided test, α = 0.05, 1-β = 90%
σ = 2hr (standard deviation)
δ = 1 hr (difference of interest)
( Z 1 / 2  Z 1  ) 
2
n

2
2
(1.960  1.282) 2
2

1
2
2
 42.04  43
Short Helpful Hints
• In humans n = 12-15 gives somewhat stable
variance
– Not about power, about stability
– 15/arm minimum good rule of thumb
• If n < 20-30, check t-distribution
• Minimum 10 participants/variable
– Maybe 100 per variable
Sample Size:
Change Effect or Difference
• Change difference of interest from 1hr to 2 hr
• n goes from 43 to 11
(1.960  1.282) 2
2
n
2
2
2
 10.51  11
Sample Size:
Iteration and the Use of t
• Found n = 11 using Z
• Use t10 instead of Z
– tn-1for a simple 1 sample
• Recalculate, find n = 13
• Use t12
• Recalculate sample size, find n = 13
– Done
• Sometimes iterate several times
Sample Size: Change Power
• Change power from 90% to 80%
• n goes from 11 to 8
• (Small sample: start thinking about using the t
distribution)
(1.960  0.841) 2
2
n
2
2
2
 7.85  8
Sample Size:
Change Standard Deviation
• Change the standard deviation from 2 to 3
• n goes from 8 to 18
(1.960  0.841) 3
2
n
2
2
2
 17.65  18
Sleep Aid Example: 2 Arms
Investigational, Control
• Original design (2-sided test, α = 0.05, 1-β = 90%, σ =
2hr, δ = 1 hr)
• Two sample randomized parallel design
• Needed 43 in the one-sample design
• In 2-sample need twice that, in each group!
• 4 times as many people are needed in this design
2( Z 1   / 2  Z 1   ) 
2
n

2
2
2(1.960  1.282) 2
2

1
2
2
 84.1  85  170 total!
Sleep Aid Example: 2 Arms
Investigational, Control
• Original design (2-sided test, α = 0.05, 1-β = 90%, σ =
2hr, δ = 1 hr)
• Two sample randomized parallel design
• Needed 43 in the one-sample design
• In 2-sample need twice that, in each group!
• 4 times as many people are needed in this design
2( Z 1   / 2  Z 1   ) 
2
n

2
2
2(1.960  1.282) 2
2

1
2
2
 84.1  85  170 total!
Aside: 5 Arm Study
• Sample size per arm = 85
• 85*5 = 425 total
– Similar 5 arm study
– Without considering multiple comparisons
Sample Size:
Change Effect or Difference
• Change difference of interest from 1hr to 2 hr
• n goes from 170 to 44
2(1.960  1.282) 2
2
n
2
2
2
 21.02  22  44 total
Sample Size: Change Power
• Change power from 90% to 80%
• n goes from 44 to 32
2(1.960  0.841) 2
2
n
2
2
2
 15.69  16  32 total
Sample Size:
Change Standard Deviation
• Change the standard deviation from 2 to 3
• n goes from 32 to 72
2(1.960  0.841) 3
2
n
2
2
2
 35.31  36  72 total
Conclusion
• Changes in the difference of interest have HUGE impacts on
sample size
– 20 point difference → 25 patients/group
– 10 point difference → 100 patients/group
– 5 point difference → 400 patients/group
• Changes in difference to be detected, α, β, σ, number of
samples, if it is a 1- or 2-sided test can all have a large impact
on your sample size calculation
2
2-Arm Study’s
4( Z 1   / 2  Z 1   ) 
TOTAL Sample Size = 2 N 
2

2
Other Designs?
Sample Size:
Matched Pair Designs
• Similar to 1-sample formula
• Means (paired t-test)
– Mean difference from paired data
– Variance of differences
• Proportions
– Based on discordant pairs
Examples in the Text
•
•
•
•
Several with paired designs
Two and one sample means
Proportions
How to take pilot data and design the next
study
Cohen's Effect Sizes
• Large (.8), medium (.5), small (.2)
• Popular especially in social sciences
• Do NOT use unless no choice
– Need to think
• ‘Medium’ yields same sample size regardless
of what you are measuring
Outline
Power
Basic sample size information
Examples (see text for more)
Changes to the basic formula/ Observational
studies
• Multiple comparisons
• Rejected sample size statements
• Conclusion and Resources
Unequal #s in Each Group
• Ratio of cases to controls
• Use if want λ patients randomized to the treatment
arm for every patient randomized to the placebo arm
• Take no more than 4-5 controls/case
n 2   n1   controls for every case
( Z 1   / 2  Z 1   ) (
2
n1 

2
2
1

2
2
/ )
K:1 Sample Size Shortcut
• Use equal variance sample size formula:
TOTAL sample size increases by a factor of
(k+1)2/4k
• Ex: Total sample size for two equal groups =
26; want 2:1 ratio
• 26*(2+1)2/(4*2) = 26*9/8 = 29.25 ≈ 30
• 20 in one group and 10 in the other
Unequal #s in Each Group:
Fixed # of Cases
• Only so many new devices
• Sample size calculation says n=13 per arm
needed
• Only have 11 devices!
• Want the same precision
• n0 = 11 device recipients
• kn0 = # of controls
How many controls?
k 
n
2 n0  n
• k = 13 / (2*11 – 13) = 13 / 9 = 1.44
• kn0 = 1.44*11 ≈ 16 controls (and 11 cases) = 27
total (controls + cases)
– Same precision as 13 controls and 13 cases (26
total)
# of Events is Important
• Cohort of exposed and unexposed people
• Relative Risk = R
• Prevalence in the unexposed population = π1
Formulas and Example
R 
R isk of event in exposed group
R isk of event in unxposed group
n1 
( Z 1 / 2  Z 1  )
2( R  1)
2
2
 #of events in unexposed group
n 2  R n1  #events in exposed group
n1 and n 2 are the num ber of events in the tw o groups
req uired to detect a relative risk of R w ith pow er 1- 
N  n1 /  1  # subjects per group
# of Covariates and # of Subjects
• At least 10 subjects for every variable investigated
– In logistic regression
– No general theoretical justification
– This is stability, not power
– Peduzzi et al., (1985) unpredictable biased
regression coefficients and variance estimates
• Principal component analysis (PCA) (Thorndike 1978
p 184): N≥10m+50 or even N ≥ m2 + 50
Balanced Designs: Easier to Find Power /
Sample Size
• Equal numbers in two groups is the easiest to
handle
• If you have more than two groups, still, equal
sample sizes easiest
• Complicated design = simulations
– Done by the statistician
Outline
Power
Basic Sample Size Information
Examples (see text for more)
Changes to the basic formula
Multiple comparisons
• Rejected sample size statements
• Conclusion and Resources
Multiple Comparisons
• If you have 4 groups
– All 2 way comparisons of means
– 6 different tests
• Bonferroni: divide α by # of tests
– 0.025/6 ≈ 0.0042
– Common method; long literature
• High-throughput laboratory tests
DNA Microarrays/Proteomics
• Same formula (Simon et al. 2003)
– α = 0.001 and β = 0.05
– Possibly stricter
• Many other formulas
Outline
Power
Basic Sample Size Information
Examples (see text for more)
Changes to the basic formula
Multiple comparisons
Rejected sample size statements
• Conclusion and Resources
No, not from your grant application…..
• Statistics Guide for Research Grant Applicants
• St. George’s Hospital Medical School Department of
Public Health Sciences
• http://wwwusers.york.ac.uk/~mb55/guide/guide14.pdf
• EXCELLENT resource
Me, too! No, Please Justify N
• "A previous study in this area recruited 150
subjects and found highly significant results
(p=0.014), and therefore a similar sample size
should be sufficient here."
– Previous studies may have been 'lucky' to find
significant results, due to random sampling
variation.
No Prior Information
• "Sample sizes are not provided because there
is no prior information on which to base
them."
– Find previously published information
– Conduct small pre-study
– If a very preliminary pilot study, sample size
calculations not usually necessary
Variance?
• No prior information on standard deviations
– Give the size of difference that may be detected in
terms of number of standard deviations
Number of Available Patients
• "The clinic sees around 50 patients a year, of
whom 10% may refuse to take part in the
study. Therefore over the 2 years of the study,
the sample size will be 90 patients. "
– Although most studies need to balance feasibility
with study power, the sample size should not be
decided on the number of available patients
alone.
– If you know # of patients is an issue, can phrase in
terms of power
Outline
Power
Basic Sample Size Information
Examples (see text for more)
Changes to the basic formula
Multiple comparisons
Rejected sample size statements
Conclusion and Resources
Conclusions:
What Impacts Sample Size?
• Difference of interest
– 20 point difference → 25 patients/group
– 5 point difference → 400 patients/group
• σ, α, β
• Number of arms or samples
• 1- or 2-sided test
Total Sample Size 2-Armed/Group/Sample Test
4( Z 1   / 2  Z 1   ) 
2
2N 

2
2
No Estimate of the Variance?
• Make a sample size or power table
• Make a graph
• Use a wide variety of possible standard
deviations
• Protect with high sample size if possible
Top 10 Statistics Questions
10. Exact mechanism to randomize patients
9. Why stratify? (EMEA re: dynamic allocation
8. Blinded/masked personnel
 Endpoint assessment
Top 10 Statistics Questions
7. Each hypothesis
 Specific analyses
 Specific sample size
6. How / if adjusting for multiple comparisons
5. Effect modification
Top 10 Statistics Questions
4. Interim analyses (if yes)
 What, when, error spending model / stopping
rules
 Accounted for in the sample size ?
3. Expected drop out (%)
2. How to handle drop outs and missing data
in the analyses?
Top 10 Statistics Questions
1. Repeated measures / longitudinal data
 Use a linear mixed model instead of repeated
measures ANOVA
 Many reasons to NOT use repeated measures
ANOVA; few reasons to use
 Similarly generalized estimating equations (GEE)
if appropriate
Analysis Follows Design
Questions → Hypotheses →
Experimental Design → Samples →
Data → Analyses →Conclusions
• Take all of your design information to a
statistician early and often
– Guidance
– Assumptions
Questions?
Resources: General Books
• Hulley et al (2001) Designing Clinical Research,
2nd ed. LWW
• Rosenthal (2006) Struck by Lightning: The
curious world of probabilities
• Bland (2000) An Introduction to Medical
Statistics, 3rd. ed. Oxford University Press
• Armitage, Berry and Matthews (2002)
Statistical Methods in Medical Research, 4th
ed. Blackwell, Oxford
Resources: General/Text Books
• Altman (1991) Practical Statistics for Medical
Research. Chapman and Hall
• Fisher and Van Belle (1996, 2004) Wiley
• Simon et al. (2003) Design and Analysis of
DNA Microarray Investigations. Springer
Verlag
• Rosner Fundamentals of Biostatistics. Choose
an edition. Has a study guide, too.
Sample Size Specific Tables
• Continuous data: Machin et al. (1998) Statistical
Tables for the Design of Clinical Studies, Second
Edition Blackwell, Oxford
• Categorical data: Lemeshow et al. (1996) Adequacy
of sample size in health studies. Wiley
• Sequential trials: Whitehead, J. (1997) The Design
and Analysis of Sequential Clinical Trials, revised 2nd.
ed. Wiley
• Equivalence trials: Pocock SJ. (1983) Clinical Trials: A
Practical Approach. Wiley
Resources: Articles
• Simon R. Optimal two-stage designs for phase
II clinical trials. Controlled Clinical Trials. 10:110, 1989.
• Thall, Simon, Ellenberg. A two-stage design
for choosing among several experimental
treatments and a control in clinical trials.
Biometrics. 45(2):537-547, 1989.
Resources: Articles
• Schoenfeld, Richter. Nomograms for calculating the
number of patients needed for a clinical trial with
survival as an endpoint. Biometrics. 38(1):163-170,
1982.
• Bland JM and Altman DG. One and two sided tests of
significance. British Medical Journal 309: 248, 1994.
• Pepe, Longton, Anderson, Schummer. Selecting
differentially expressed genes from microarry
experiments. Biometrics. 59(1):133-142, 2003.
Regulatory Guidances
•
•
•
•
•
ICH E9 Statistical principles
ICH E10: Choice of control group and related issues
ICH E4: Dose response
ICH E8: General considerations
US FDA guidance and draft guidance on drug
interaction study designs (and analyses), Bayesian
methods, etc.
– http://www.fda.gov/ForIndustry/FDABasicsforInd
ustry/ucm234622.htm
Resources: URLs
• Sample size calculations simplified
– http://www.jerrydallal.com/LHSP/SIZE.HTM
• Stat guide: research grant applicants, St. George’s
Hospital Medical School
(http://www-users.york.ac.uk/~mb55/guide/guide.htm)
– http://tinyurl.com/7qpzp2j
• Software: nQuery, EpiTable, SeqTrial, PS
(http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/PowerSampleSize)
– http://tinyurl.com/zoysm
• Earlier lectures
Various Sites by Statisticians
• www.pmean.com/category/HumanSideStatistics.html
• www.pmean.com/category/RandomizationInResearch.html
• www.pmean.com/category/SampleSizeJustification.html
• http://www.cs.uiowa.edu/~rlenth/Power/